Roller contact arrangement for mechanical current converters or other equipment of similar construction



Dec. 23, 1958 GOTTWALD 2,866,020

ROLLER CONTACT ARRANGE T FOR MECHANICAL CURRENT CONVERTERS OR OTHER EQUIPMENT OF SIMILAR CONSTRUCTION Filed June 18. 1953 3 Sheets-Sheet 1 26 2a 32 R 30 20 a5 R ll Dec. 23, 1958 w. GOTTWALD 2,366,020

ROLLER CONTACT ARRANGEMENT FOR MECHANICAL CURRENT CONVERTERS OR OTHER EQUIPMENT OF SIMILAR CONSTRUCTION Filed June 18, 1953 3 Sheets-Sheet 2 Dec. 23, 1958 w. GOTTWALD 2,866,020

' ROLLER CONTACT ARRANGEMENT FOR MECHANICAL CURRENT CONVERTERS OR OTHER EQUIPMENT OF SIMILAR CONSTRUCTION Filed June 18. 1953 s Sheets-Sheet 3 92 The .lnvcnor:

United States Patent CHANICAL CURRENT CONVERTERS OR OTHER EQUIPMENT OF SIMILAR CONSTRUCTION Walter Gottwald, 'Ratingen, near Dusseldorf, Germany, assignor to August Hamilton Schilling, Atherton, Calif.

Application June 18, 1953, Serial No. 362,533 Claims priority, application Germany July 8, 1952 11 Claims. (Cl. 200-25) This invention relates to a roller contact arrangement for mechanical current converters or other equipment of similar construction having at least .one roller contact travelling on one or several especially cylindrically shaped contact surfaces, and wherein said roller contacts are pressed against the contact surface by centrifugal forces.

It is known in the case of roller contacts for current converters which travel on the inner side of a cylindrical contact surface to journal such roller contacts in rotating carrier arms with a certain amount of play,

whereby .under the influence of the centrifugal force, and

each according to its weight and the velocity with which it moves on the contact surface, the rollers produce a .certain pressure between them and the contact surface.

This contact pressure, however, must be proportioned in installations of the type referred to in such manner that on the, one hand a good transfer of current is secured from the electrical standpoint, and on the other .hand the contact pressure is not so high that the mechanical wear .ofthe, arrangement, which must operate without interruption for long periods, is not too great so as not to reduce to an undesirable degree the service life of the rollers. In order to attain this with the known construction, the weight of the rollers must bear a, definite relationship to the speed of rotation on the contact surface. It is not always easy to fulfill this requirement, if the construction is'to be economical. If, for instance, in a mechanical rectifier an arrangement is selected having only one travelling roller contact, such roller must necessarily revolve with high speed on the cylindrical contact surface in order to effect the required switching in synchronism with the alternating current. Therefore the roller must be comparatively small and of low weight, as the centrifugal force is great and thus with a rather small roller the required contact pressure is obtained. However, it is doubtful that such a comparatively small roller will be able to stand the desired current load and that its speed of rotation can be maintained within the limits permissible for continuous operation. If, on the other hand, an arrangement is employed with a plurality of current switching rollers, which permits a desirable reduction in the speed, since plural switching occurs at the annular contact during each revolution of the arms, bigger and heavier rollers become necessary in order to obtain the required centrifugal force for production of the necessary contact pressure for securing a satisfactory current transfer. Aside from the fact that the same design of rollers cannot be used in both cases, in the last mentioned case it becomes necessary under certain circumstances to make the arrangement undesirably bulky by reason of the larger rollers. In addition, large rollers decrease undesirably the speed of separation of the contacts, and this decrease must be balanced again by an increase in size, this time by enlargement of the contact path (roller circumference) The foregoing shows clearly the difficulties and dis- '2 advantages resulting from coupling the mechanical and electrical arts in a mechanical converter.

The present invention has for its object the provision of a converter wherein such difficulties and disadvantages can be completely avoided.

Another object of the invention is 'to'provide an arrangement of the type referred to above having means for adjusting the magnitude of the centrifugal force which produces the contact pressure.

In accordance with the present invention, the size and weight of the roller contacts can be designed without regard to the requirements as to the contact pressure to be produced, since the centrifugal force producing the contact pressure can be determined or adjusted separately for each speed of revolution.

Especially advantageous conditions result, if according to a further feature of the invention, the roller contact is arranged on a lever pivoted eccentrically on the driving shaft and movable transversely thereto. Thus, a weight can be arranged on the lever, and preferably this weight is shiftably mounted on said lever. Also, the lever can be a double-armed lever, so that the rollers can be arranged on the one arm and the weight on the other. Weights are advantageously employed which consist of one or more screw nuts adjustable on said lever. By using two screw nuts, one of them may serve as lock nut. In the use of rollers running on the inner surface of an annular path, the weight is advantageously so arranged and proportioned that it serves as a counterweight for the rollers which are under the influence of the centrifugal force. This assumes that the rollers and counterweight are located on opposite arms of the lever, so that the centrifugal force of the rollers is decreased by the counterweight. Where it may be required, the weight can be arranged on the lever arm carrying the roller, so that the centrifugal force is increased. If, on the other hand, rollers are employed which run on the outer surface of an annular path, the weight is so arranged and proportioned that by its centrifugal force it presses the rollers against the contact path and thus exceeds the centrifugal force of the rollers.

The roller contact arrangement according to the invention may be used with a great variety of installations having roller contacts. The arrangement can be used, for instance, with mechanical converters in which one or more roller contacts cooperate with contact surfaces consisting of circular or cylindrical segments. Said arrangement may also be used with such mechanical converters or rectifiers in which circular contact paths are formed by one or more contact rollers, at least one of such paths consisting of circular segments which are preferably supplemented by insulating segments to form a complete cylinder. The arrangement according to the invention may also be used with interrupted contact paths or with contact paths provided with gaps, as the moment of inertia of the mass of revolving parts can easily be made large enough to permit an uninterrupted run of the contact rollers.

In accordance with a further development of the invention, the arrangement of contact rollers disposed on levers makes it possible to provide in simple manner a construction which permits lifting of the contact rollers from the contact surface even during operation in order to interrupt the electrical operation before the apparatus has come to a mechanical standstill. Such an arrangement can' be provided, for instance, with catch members which act on the roller-carrying levers. For this purpose, an energy accumulator, such as an arrange ment of springs, can be released by means of an electromagnet or other actuating device and thereuponcause the lifting of the contact rollers.

For a fuller understanding of the nature and the interior rollers or electrodes and one exterior contact path or electrode which is contacted by the interior electrodes and is of annular shape;

Fig. 2 shows the current converter according to Fig. 1 diagrammatically in perspective and indicates the current connections and the required insulating pieces;

Fig. 3 shows a modification wherein the current converter is provided with two exterior electrodes and one annular interior electrode;

Fig. 4 illustrates in axial section the mode of construction of a current converter wherein the electrodes are designed to be removed or lifted during operation; while Fig. 5 shows the current converter according to Fig. 4 in front elevation.

Referring to the drawing, the two contact rollers 1 and 2 roll on a contact path 3 represented by a circle. The design of this contact path is immaterial from the electrical standpoint, since according to the invention only the mechanical effect, i. e., the realization of the desired contact pressure is important. The contact path 3, for instance, may consist of individual contacting segments which alternate with segments made of insulating material. Also two contact paths may be arranged spatially separated from but in parallel to each other, one overlying the other so that in the drawing it covers the latter. Both paths may be connected with one another by the rollers 1 and 2, either both paths or only one of them being composed of individual segments. By means of their axles 4 or 5, rollers 1 and 2 are journalled on levers 6 and 7 which in turn are attached pivotally to the driver 10. The driver 10 is rigidly connected to the shaft 11, driven, for instance, by a synchronous motor. The levers 6 and 7 are formed as threaded rods at 12 and 13 on which heavy threaded nuts 14, 15 and 16, 17 are arranged which serve as counterweights for the rollers 1 and 2. Each threaded nut of the pairs 14, 15, and 16, 17 acts as a jam nut to fix the adjusted position against displacement.

When shaft 11 rotates, the rollers 1 and 2 roll in the direction of the arrows 18 and 19 on the contact path 3 while rotating on their own axles 4 and 5. It can be seen that the so generated centrifugal forces press the rollers 1 and 2 against the path 3 and that by selection, arrangement and adjustment of the weights 14 to 17 the desired degree of contact pressure can be determined which, in this case, corresponds to the difference of the effective centrifugal forces. By arranging the mass weights 14 to 17 on the side of the levers on which the rollers 1 and 2 are disposed, the contact pressure can be made to correspond to the "sum of the efiective centrifugal forces.

A further illustration of the invention is presented in Fig. 2 of the drawing which shows in perspective the basic design of a roller current converter for the conversion of rotary current to direct current. There are shown two contact sufaces 20 and 21 which are connected by the rollers 22 and 23. The rollers 22, 23 are designed in the same manner as the rollers 1 and 2 of Fig. 1; however, for the sake of clarity, these are not shown in detail. The shaft 11, whose geometric axis is indicated at 2424 in Fig. 2, is rotated in the direction of the arrow 25 by a synchronous motor (not shown). Thus rollers 22 and 23 rotate on their own axles in the directions of the arrows 26 and 27, respectively, and roll in the direction of the arrows 28 and 29 on the contact surfaces 20 and 21. The contact surfaces 20, 21 are formed by rings 30 and 31, ring 30 being a closed metal ring lined at proper intervals with segments of insulating material 32; while the ring 31 consists of three metal pieces 33 which are interrupted by pieces 34 made of insulating material. The three phase lines R, S, T of a rotary current power line are connected to the metal segments 33 of the ring 31. The D. C. power line 35 is led off from the closed ring 30. Each of the metal segments extends for 90, while each of the insulating segments 34 extends for 30 of the annulus. As an example, it is assumed that a SO-cycle rotary current is to be converted to direct current. Then the speed of the synchronous motor and that of the rollers 22 and 23 on the roller paths 20 and 21 around the axis 24'-24 is 1500 R. P. M. With the revolution of the rollers and by contact with the rollers 22, 23, the segments 33 will be connected in succession with the D. C.-collector ring 30 in such a manner that current is supplied to it only in one direction and thus a three-phase commutation operation is realized. The insulating segments 32 have the advantage that the switching period can be adjusted by relative rotary adjustment of the rings 30 and 31.

It is understood that roller arrangements according to Fig. 1 can also be used successfully with designs of roller current converters which differ from the type shown in Fig. 2; and provided only that rollers are employed which travel on contact paths, the mechanism is present in which the present invention can be successfully embodied'for adjusting the magnitude of the contact pressure.

Fig. 3 shows a type of roller contact arrangement wherein the rollers 36, 37 are disposed on the outside of a contact surface 38. The same measures are applicable to this arrangement as have been shown in connection with the embodiment of the invention shown in Fig. 1- for making the contact pressure adjustable with which the rollers 36, 37 engage the contact surface 38. Accordingly, the rollers 36, 37 are again guided for rotation on their .axles 41, 42 by fork-shaped levers 39, 40. The necessary drive is effected by shaft 44 through the driver 43 connected to the levers 39, 40. The fork-shaped levers 39, are shaped at one end as threaded rods on which threaded nuts 47, 48 and 49, 50 are arranged. The weight and adjustment of these threaded nuts are so selected that with a specific speed of the driving shaft 44 and with the given weight of the rollers 36, 37, predetermined contact pressure of the rollers 36, 37 against the contact surface 38 is obtained. The contact surface 38 may be constructed in any suitable manner. In the embodiment of the invention shown in Fig. 3, the contact surface has been designed-like the contact ring 30 in Fig. 2, i. e., inserts 51, 52, 53 consisting of insulating material are provided which have the same effect as the insulation pieces 32 in Fig. 2, as explained in detail hereinabove.

Figs. 4 and 5 show an embodiment of the invention wherein the removal or lifting of the rollers from the contact surface during operation is made possible, Fig. 4 corresponding in part to a section along the line IV-IV of Fig. 5.

In Figs. 4 and 5 the numeral 54 designates the shaft driven by a synchronous motor (not shown) and operating to effect rotation of the roller contacts. For this purpose there is again provided on the shaft a driver 55 having axle journals 56, 57 on which levers 60, 61 supporting the rollers 58, 59 are pivotally arranged. The rollers 58, 59 are received in the fork-shaped ends of the levers 60, 61 and contact the inner contact surface 62 of the contact ring 63 which may be designed, for instance, in the same manner as the contact ring 30 in Fig. 2, there being provided at 64, 65 and 66 in the contact ring 63 three segments consisting of insulating material. So far as these features are concerned, the design of the apparatus shown in Figs. 4 and 5 does not differ from that shown in Figs. 1 and 2. However, the construction of Figs. 4 and 5 is provided with means for removing the rollers 58, 59 from the contact ring 63 during operation. As can be seen in Fig. 5, these means consist of two wedge pieces 67, 68 which are brought to bear on levers 60, 61 in such manner that the points of engagement are opposite to the arrangement of the rollers '58, 59 with respect to the centers of rotation 56, 57. The wedge pieces are arranged on adjustable rods 69, 70 carried by a plate 71 which in its turn is axially shiftable on the shaft 54/ By the provision of wedges or similar means plate 71 is made axially movable despite its rotary movement on the shaft 54. For the axial displacement of plate 71 on shaft 54 a sleeve 72 having a ball bearing 73 is provided. The outer race ring of the ball bearing is held by a forked member 74 having extensions at 75 and 76. These extensions are fixed at 77 and 78 in slots of guide pistons which are provided in the piston rods 79 and 80. The piston rods terminate at 8'1 and 82 in adjustable pistons which are 83 and 84 and are under the influence of return springs 85, 86 on the one hand, and of a hydraulically or pneumatically effective working medium which is admitted at 87, 88 on the other. The pressure of the working medium is capable of overcoming the counter-effect of the return springs 85, 86. This means that the Wedge pieces 67, 68 in Fig. 4 can increase the radial distance of the levers 60, 61. Thereby, as is shown in Fig. 5, forces can be produced which are greater than the centrifugal forces of the rollers 58, 59 relative to the centrifugal forces of the adjustable counterweights 89, 90 and 91, 92, respectively, which are effective in the opposite direction. The measures shown thus permit without further difiiculties the removal of the rollers 58, 59 from the contact path 63 at any desired time by admitting the working medium at 87 and 88. If the cylinders 83, 84 are relieved of the pressure of the working medium, the return springs 85, 86 return the rollers 58, 59 with the aid of the counerweights 89 to 92 to the position in which engagement of the contact surfaces 58, 59 and 62 is assured.

It will be evident from the foregoing that in all of the embodiments of the invention, the magnitudes of the mass of each roller and of its associated weight (14, 15 or 16, 17 or 89, 90 or 91, 92), and their relative arrangement, are such that in the normal operating condition of the converter, i. e., at its rated speed, there will be a,

force operating to press the roller against the circular contact path. Expressed differently, and assuming that a force which presses the roller against thecontact path is positive, while one that acts to separate the two is negative, then in all cases the algebraic sum of the forces acting on the roller at the rated speed of the converter is 7 positive.

I claim:

1. Contact current converter having terminals for electric currents at least one of which is adapted to receive alternating current, and comprising, in combination, an electrode and a counter-electrode having contact surfaces arranged for engagement with each other along a curvilinear path, certain of said surfaces being connected with said terminals, one of said electrodes being in the form of a revolvably mounted roller which is adapted to roll on the cooperating curvilinear contact surface, means for bodily rotating such roller about an axis spaced from the axis of revolution of the roller, and at theconstant speed in synchronism with the frequency of the alternating current, to cause such roller to roll continuously on the other electrode, the centrifugal forces arising fro-m the rotation of said roller electrode about said axis operating to provide continuous contact pressure between the electrodes, and an additional mass associated with the rotating roller electrode for generating a centrifugal force which opposes the forces acting on the roller, the net force acting on the roller operating to hold the same in contact with the contact surface at any given circumferential velocity and being determined at any speed solely by the relative magnitude of said mass and of the mass of the roller, and by their relative arrangement.

2. Contact current converter having terminals for electric currents at least one of which is adapted to receive alternating current, and comprising, in combination, an electrode and a counter-electrode having contact surfaces arranged for engagement with each other along a curvilinea: path certain of said surfaces being connected with guided in adjustable cylinderssaid terminals, one of said electrodes being in the form of at least one revolvably mounted contact roller which is adapted to roll on the cooperatingcurvilinear contact surface, means for causing said contact roller to roll continuously on the other electrode, said means comprising a drive shaft rotating at a constant speed in synchronism with the frequency of the alternating current, a driver rotated by said shaft, a lever pivotally mourited on said driver, said contact roller being mounted on said lever and bearing against the counter-electrode, and an additional mass disposed on said lever for modifying the contact pressure determined by the centrifugal force arising from the rotation of the contact roller, the net force acting on the roller operating to hold the same in contact with the contact surface at any given circumferential velocity and being determined at any speed solely by the relative magnitudes of saidmass and of the mass of the roller, and by their relative arrangement.

3. A converter as defined in claim 2, wherein the pivot of the lever on the driver is arranged bet-ween the contact roller and the additional mass.

4. Contact current converter having terminals for electric currents at least one of which is adapted to receive alternating current, and comprising, in combination, an electrode and a counter-electrode having contact surfaces arranged for engagement with each other along a curvilinear path, certain of said surfaces being connected with said terminals, one of said electrodes being in the form of an at least one revolvably mounted contact rollerwhich is adapted to roll on the cooperating curvilinear contact surface, means for causing said contact roller to roll continuously on the other electrode, said means comprising a drive shaft rotating at a constant speed in synchronism with the frequency of the alternating current, a driver rotated by said shaft, a lever pivotally mounted on said driver, said contact roller being mounted on said lever, the counter-electrode being of annular shape and said contact roller continuously engaging the inner surface thereof during the operation of the converter, and an additional mass arranged on said lever for reducing the contact pressure between the electrodes caused by the centrifugal force generated by the rotation of the contact roller, the net force acting on the roller operating :to hold the same in contact with the contact surface at any given circumferential velocity and being determined at any speed solely by the relative magnitudes of said mass and of the mass of the roller, and by their relative arrangement.

5. Contact current converter having terminals for electric currents at least one of which is adapted to receive alternating current, and comprising, in combination, an electrode and a counter-electrode having contact surfaces arranged for engagement with each other along a curvilinear path, certain of said surfaces being connected with said terminals, one of said electrodes being in the form of an at least one revolvably mounted contact roller which is adapted to roll on the cooperating curvilinear contact surface, means for causing said contact roller to roll continuously on the other electrode, said means comprising a drive shaft, a driver rotated by said shaft, a'lever pivotally mounted on said driver intermediate its ends, said contact roller being mounted on said lever, said counterelectrode being arranged inwardly of the rollerelectrodc, and said roller electrode being arranged externally of the counter-electrode and rolling on the outer surface thereof, and an additional mass arranged on said lever on the opposite side of the pivot thereof, the net force acting on the roller operating to hold the same in contact with the contact surface at any given circumferential velocity and being determined at any velocity solely by the relative magnitudes of said mass and of the mass of the roller, and by their relative arrangement.

6. Contact current converter having terminals for electric currents at least one of which is adapted to receive alternating current, and comprising, in combination, a

pair of axially spaced annular electrodes, one of which is composed of insulated curvilinear conducting segments electrically connected with the alternating current terminals, a centrally mounted drive shaft adapted to be rotated in synchronism with the frequency of the alternating current, a driver rigidly connected with the shaft and extending in opposite directions therefrom, levers pivotally mounted intermediate their ends on the diagonally opposite ends of the driver, a contact roller revolvably mounted at one end of each lever and in diagonally opposite relation with reference to said shaft, said rollers being bodily rotated by said shaft by way of the driver and levers and in rolling engagement with said axially spaced annular electrodes, and an additional mass adjusta bly positioned adjacent to the end of each lever opposite its roller and operating to reduce the centrifugal contact pressure between the roller and the annular electrodes, the net force acting on the rollers operating to hold the same in contact with the annular electrodes at any given circumferential velocity and being determined in each case solely by the relative magnitudes of the additional mass and of the mass of the roller associated therewith, and by their relative arrangement.

7. Contact current converter having terminals for electric currents at least one of which is adapted to receive alternating current, and comprising, in combination, an electrode and a counter-electrode having contact surfaces arranged for engagement with each other along a curvilinear path, certain of said surfaces being connected with said terminals, one of said electrodes being in the form of at least one revolvably mounted contact roller which is adapted to roll on the cooperating curvilinear contact surface, means for causing said contact roller to roll on the other electrode, said means comprising a drive shaft, a driver rotated by said shaft, a lever pivotally mounted on said driver, said contact roller being mounted on said lever and bearing against the counter-electrode, and an additional mass disposed on said lever for modifying the contact pressure determined by the centrifugal force arising from the rotation of the contact roller about said drive shaft, the relative magnitudes of said mass and of the mass of the roller and their relative arrangement alone acting to produce the net positive force acting on the roller to hold the same in contact with the cooperating contact surface at any given circumferential velocity, the distance of the center of gravity of said additional mass from the center of the pivot of the lever on the driver being adjustable.

8. Contact current converter having terminals for electric currents at least one of which is adapted to receive alternating current, and comprising,.in combination, an electrode and a counter-electrode having con-tact surfaces arranged for engagement with each other along a curvilinear path, certain of said surfaces being connected with said terminals, one of said electrodes being in the form of at least one revolvably mounted contact vroller which is adapted to roll on the cooperating curvilinear contact surface, means for causing said contact roller to roll on the other electrode, said means comprising a drive shaft, a driver rotated by said shaft, a lever pivotally mounted on said driver, said contact roller being mounted on said lever and bearing against the counter-electrode, and an additional mass disposed on said lever for modifying the contact pressure determined by the centrifugal force arising from the rotation of the contact roller about said drive shaft, the relative magnitudes of said mass and of the mass of the roller and their relative arrangement alone acting to produce the net positive force acting on the roller to hold the same in contact with the cooperating contact surface at any given circumferential velocity, said lever being formed in part as a threaded rod, and said additional mass being in the form of a threaded member movably mounted on said threaded rod.

9. Contact current converter having terminals for electric currents at least one of which is adapted to receive alternating current, and comprising, in combination, an electrode and a counter-electrode having contact surfaces arranged for engagement with each other along a curvilinear path, certain of said surfaces being connected with said terminals, one of said electrodes being in the form of at least one revolvably mounted contact roller which is adapted to roll on the cooperating curvilinear contact surface, means for causing said contact roller to roll on the other electrode, said means comprising a drive shaft, a driver rotated by said shaft, a lever pivotally mounted on said driver, said contact roller being mounted on said lever and bearing against the counter-electrode, and an additional mass disposed on said lever for modifying the contact pressure determined by the centrifugal force arising from the rotation of the contact roller about said drive shaft, the relative magnitudes of said mass and of the mass of the roller and their relative arrangement alone acting to produce the net positive force acting on the roller to hold the same in contact with the cooperating contact surface at any given circumferential velocity, said lever being formed in part as a threaded rod, said additional mass comprising a nut threaded on said rod, and a second nut likewise forming part of said additional mass and acting to lock the first nut on said threaded rod.

10. Contact current converter having terminals for electric currents at least one of which is adapted to receive alternating current, and comprising, in combination, an electrode and a counter-electrode having contact surfaces arranged for engagement with each other along a curvilinear path, certain of said surfaces being connected with said terminals, one of said electrodes being in the form of at least one revolvably mounted contact roller which is adapted to roll-on the cooperating curvilinear contact surface, means for causing said contact roller to roll continuously on the other electrode, said means comprising a drive shaft rotating at a constant speed in synchronism with the frequency of the alternating current, a driver rotated by said shaft, a lever pivotally mounted on said driver, said contact roller being mounted on said lever and bearing against the counter-electrode, an additional mass disposed on said lever for modifying the contact pressure determined by the centrifugal force arising from the rotation of the contact roller, the centrifugal force acting on the additional mass opposing the force acting on the roller but at no time causing separation of the electrodes during the operation of the converter, the relative magnitudes of said mass and of the mass of the roller and their relative arrangement acting to produce a net positive force on the roller operating to maintain contact between the roller and the counter-electrode at the constant speed of the converter, and means for rotating the lever during the operation of the converter, to lift the roller from the counter-electrode against the action of the said net positive force.

11. Contact current converter having terminals for electric currents at least one of which is adapted to receive alternating current, and comprising in combination, an electrode and a counter-electrode having contact surfaces arranged for engagement with each other along a curvilinear path, certain of said surfaces being connected with said terminals, one of said electrodes being in the form of at least one revolvably mounted contact roller which is adapted to roll on the cooperating curvilinear contact surface, means for causing said contact roller to roll continuously on the other electrode, said means comprising a drive shaft rotating at a constant speed in synchronism with the frequency of the alternating current, a driver rotated by said shaft, a lever pivotally mounted on said driver, said contact roller being mounted on said lever and bearing against the counter-electrode, an additional mass disposed on said lever for modifying the contact pressure determined by the centrifugal force arising from the rotation of the contact roller, the centrifugal force acting on the ad? ditional mass opposing the force acting on the roller but at no time causing separation of the electrodes during the operation of the converter, the relative magnitudes of said mass and of the mass of the roller and their relative arrangement acting to produce a net positive force on the roller operating to maintain contact between the roller and the counter-electrode at the constant speed of the converter, the lever being provided with an inclined surface disposed opposite the roller with respect to the pivot of the lever, a wedge engageable with said 10 inclined'surface to effect rotation of the lever and thereby cause lifting of the roller from the counter-electrode,

and means for moving the wedge during the operation of the converter.

References Cited in the file of this patent UNITED STATES PATENTS 862,041 Wilkinson July 30, 1907 862,127 Auel Aug. 6, 1907 1,177,548 Southwick Mar. 28, 1916 1,515,531 Wilber Nov. 11, 1924 1,558,304 Smulski Oct. 20, 1925 1,855,703 Cloud Apr. 26, 1932 2,133,980 Fowler Oct. 25, 1938 2,483,115 Wall Sept. 28, 1949 

